Information can be displayed on sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays (LCDs). Magnetic sheet materials can carry magnetically writable areas for ticketing or financial information, but magnetically written data is not visible.
Flat panel LCDs use two transparent glass plates as substrates. In a typical embodiment, such as one set forth in U.S. Pat. No. 5,503,952, a set of electrical traces is sputtered in a pattern of parallel lines that form a first set of conductive traces. A second substrate is similarly coated with a set of traces having a transparent conductive coating. Coatings are applied and the surfaces rubbed to orient liquid crystals. The two substrates are spaced apart and the space between the two substrates is filled with a liquid crystal material. Pairs of conductors from either set are selected and energized to alter the optical transmission properties of the liquid crystal material. Such displays are expensive.
Fabrication of flexible, electronically written display sheets using conventional nematic liquid crystals materials is disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent indium-tin-oxide (ITO) conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be thin glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential is applied to opposing conductive areas to operate on the liquid crystal material and expose display areas. The display uses nematic liquid crystal materials, which ceases to present an image when de-energized. Privacy windows are created from such materials using the scattering properties of conventional nematic liquid crystals. Nematic liquid crystals require continuous electrical drive to remain transparent.
U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a chiral nematic liquid crystal (cholesteric liquid crystal) in polymeric domains contained by conventional patterned glass substrates. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal conic state. Chiral nematic material has two stable slates and can maintain one of the stable states in the absence of an electric field. Consequently, chiral nematic displays have no limit on the number of lines that can be addressed. U.S. Pat. Nos. 5,251,048 and 5,644,330 disclose various driving methods to switch chiral nematic materials between its stable states. However, the update rate of these displays is far too slow for most practical applications. Typically, the update rate was about 10-40 milliseconds per line. It would take a 10-40 seconds to update a 1000 line display.
U.S. Pat. Nos. 5,748,277 and 6,154,190 disclose fast driving schemes for chiral nematic displays, which are called dynamic driving schemes. The dynamic driving schemes generally consist of preparation step, pre-holding step, selection step, post-holding step, and evolution step. Those fast driving schemes require very complicated electronic driving circuitry. For example, all column and row drivers must output bi-polar and multiple level voltages. During the image writing, due to pipeline algorithm, there is an undesirable black bar shifting over the frame. U.S. Pat. No. 6,268,840 discloses a unipolar waveform drive method to implement the above-mentioned dynamic driving schemes. However, because the amplitude of voltages required in the preparation step, the selection step, and the evolution step are distinct, both column and row drivers are required to generate multilevel unipolar voltages, which is still undesirable.
Kozachenko et al. (Hysteresis as a Key Factor for the Fast Control of Reflectivity in Cholesteric LCDs, Conference Record of the IDRC 1997, pp. 148-151), Sorokin (Simple Driving Methods for Cholesteric Reflective LCDs, Asia Displays 1998, pp. 749-752), and Rybalochka et al. (Dynamic Drive Scheme for Fast Addressing of Cholesteric Displays, SID 2000, pp. 818-821; Simple Drive scheme for Bistable Cholesteric LCDs, SID 2001, pp. 882-885) proposed U/√{square root over (2)} and U/√{square root over (3/2)} dynamic driving schemes requiring only 2-level column and row drivers, which output either U or 0 voltage. These drive schemes do not produce undesirable black shifting bars, instead, they cause the entire frame to go black during the writing. However, as their names suggest, they can be applied only to those cholesteric liquid crystal displays with very specific electrooptical properties, such as Uholding=Uevolution=U/√{square root over (2)} for the U/√{square root over (2)} dynamic drive scheme, or Uholding=Uevolution=U/√{square root over (3/2)} for the U/√{square root over (3/2)} dynamic drive scheme, where Uholding and Uevolution are effective voltages (root mean square voltages) of their holding step and evolution step, respectively. Because of this limit, many cholesteric liquid crystal displays either cannot be driven by these schemes, or can be driven only with compromising contrast and brightness. Therefore, there is a need for a simple, low cost, and fast drive scheme for cholesteric liquid crystal displays without sacrificing their contrast and brightness. In addition, the prior art schemes do not teach how to achieve multiple gray levels by using a 2-level voltage driving method.